SANTA ROSA, Calif. — The historically deadly wildfires ravaging Northern California regained momentum Wednesday as winds whipped back up, pushing blazes through parched hills and vineyards and prompting more evacuations from an arc of flames that has killed at least 21 people, destroyed more than 2,000 buildings and battered the region’s renowned wine-growing industry.

Fires advanced overnight toward populated areas in flame-battered Sonoma County, prompting officials to order a fresh round of mandatory evacuations — some of which were announced by deputies “running toward the fire, banging on doors, getting people out of their houses,” said Misti Harris, a Sonoma County Sheriff’s Office spokeswoman.

Nearly two dozen large fires have been burning in the northern part of the state; on Wednesday, Cal Fire Chief Ken Pimlott said he’s worried that “several of these fires will merge.”

“This is a serious, critical, catastrophic event,” Pimlott said at a news briefing, where he made a grim but expected announcement: The death toll from the fires has increased, from 17 to 21.

Officials still expect that figure to rise: Sonoma County Sheriff Robert Giordano said Wednesday that authorities have located more than 100 people who were reported missing — but that 560 still remain unaccounted for.

Deputies have not been able to reach most of the areas called “hot zones” that were immolated in the firestorm, he said.

It’s unclear if those who are missing have been harmed, or are simply unable to reach friends and families, as fires have disabled much of the communication system in the region. Officials are hoping to find majority of them Wednesday, Giordano said.

The fires grew overnight as conditions worsened and had torched a combined 70,000 acres by Wednesday morning, according to Cal Fire.

Evacuation zones in the county will remain off limits, partly to limit the possibility of looting, which Giordano said has resulted in three arrests. He doubts residents will be allowed to return to their homes this week.

“If you have a place to go, go; you don’t need to be here,” Giordano said, adding later: “I can’t stress this enough. If you’re in an evacuation zone, you cannot come home.”

The fast-moving flames have swept through densely populated neighborhoods over the past two days, causing residents to flee from homes in the middle of the night as smoke filled their rooms.

High winds that whipped up 22 large wildfires had faded earlier Tuesday and humidity increased, assisting an operation that has drawn resources from throughout the state and neighboring Nevada. But the sharp northern wind, known as a Diablo, returned, allowing only a brief window for firefighters to carve clearings in place to stop the fires from spreading to vulnerable populated areas.

The National Weather Service expects “red-flag” conditions — including wind gusts up to 40 mph — to remain until Thursday in the North Bay Area, which includes Sonoma and Napa counties.

As a thick haze coated the sky and settled into the region’s canyons and valleys, state officials remained focused on rescue and containment.

On Wednesday morning, as weary firefighters attempted to control the fires on the front lines, dozens of fire crews from cities as far away as Bakersfield, more than 300 miles to the south, were briefed at a makeshift command center on the deteriorating conditions.

More than 25,000 people have fled homes from seven counties north of San Francisco, filling dozens of shelters that state officials had hoped to consolidate in the coming days to provide more-efficient services. Many left houses with nothing, and officials acknowledged Tuesday that it could be weeks before some are able to return to what is left. In Sonoma County, 5,000 people have taken refuge in 36 shelters as of Wednesday morning, officials said.

The scope of the damage prompted President Trump on Tuesday to approve federal emergency assistance to California, agreeing to a request made by Gov. Jerry Brown (D). The declaration, announced by Vice President Pence during a visit to the state’s Office of Emergency Services near Sacramento, provides immediate funds for debris clearing and supplies for evacuation centers, among other aid.

The fires are the most destructive in what already has been a severe wildfire season for California and much of the West, where more than 8 million acres have been charred this year. In his letter to Trump, Brown said that nearly 7,500 fires have flared in California this year. Ten of them have prompted him to declare a state of emergency.

The cause of the fires, which flared overnight Sunday and blew swiftly through 170,000 acres in the following days, was unknown and likely to remain so for some time.

Pimlott, the Cal Fire chief, said the possibility that a lightning strike started the fires was “minimal.” In California, he said, 95 percent of wildfires are started by people, inadvertently or intentionally.

“This is just pure devastation and it’s going to take us a while to get out and comb through all of this,” he said.

State officials said that firefighters planned to clear lines between the Atlas Fire and the city of Napa, and between the Tubbs Fire and the city of Santa Rosa — the largest in Sonoma County and gateway to the wine-tourism industry.

Those barriers would protect the areas from the south with the expectation that winds will shift back to the north in the days ahead.

Officials said the idea, in the case of the Tubbs Fire, was to prevent a “reburn” of Santa Rosa.

For Dylan Sayge, the original burn was devastating. He and his roommates were awake early Monday morning when they noticed an unusual sight outside their $1,600-a-month rental home in the Coffey Park neighborhood of Santa Rosa.

“We realized ash was falling from the sky,” said Sayge, 23, a musician who works at Trader Joe’s.

Soon after, online, they learned that a fast-moving fire had jumped Highway 101, propelled by howling winds. The power flickered and an explosion followed as a transformer blew nearby. They grabbed their three dogs — Cash, Willie and Shorty — and their cat, Apollo. Sayge packed up baby pictures and musical instruments.

They headed out in three cars and into a traffic jam. Sayge left behind a 1998 Ford Taurus that he had just been given as a gift. The dense smoke clouded visibility. He eventually made it to a friend’s home in Fairfax, down the road in Marin County.

The next day, he learned that the house was gone, the Taurus a charred husk.

The disruption to daily life in a region known as a calm, sometimes intoxicating, tourist destination was immense.

The 100,000 acres of vineyards — the focal point of California’s wine industry and the tourism business built around — remained threatened and, in some cases, damaged or destroyed. The extent remained unclear.

In Healdsburg, a quaint town known to tourists for its wine tasting, food and antiques, the cast was dystopian.

Smoke as thick as fog shielded the sky. On the hillside, houses burned unattended with stretched-thin firefighters busy elsewhere. The wooden guardrails along Highway 101 — one of the state’s most prominent north-south arteries — smoldered after burning the night before.

More than a dozen schools were shuttered in the seven counties most affected by fires, and damage to the power grid meant that everything from charging cellphones to pumping fuel was curtailed.

Nearly 80 cell towers have been damaged or destroyed, complicating efforts by even those with a charged battery to contact relatives or call for emergency assistance. The National Guard plans to bring in communications equipment to bolster the network, which state emergency officials called a priority.